1. Field of the Invention
The present invention relates to cold cathode fluorescent lighting (CCFL), and particularly to a method of providing a controlled start-up mode.
2. Description of the Related Art
Liquid crystal displays (LCDs) are well known in the art of electronics. One of the largest power consuming devices in a notebook computer is the backlight for its LCD. The LCD typically uses a cold cathode fluorescent lamp (CCFL) for backlighting. However, the CCFL requires a high voltage AC supply for proper operation. Specifically, the CCFL generally requires 600 Vrms at approximately 50 kHz. Moreover, the start-up voltage of the CCFL can be twice as high as its normal operating voltage. Thus, over 1000 Vrms is needed to even initiate CCFL operation.
In optimal applications, the battery in the notebook computer must generate the high AC voltages required by the CCFL. To increase valuable battery life, those skilled in the art strive to provide an efficient means to convert this low voltage DC source into the necessary AC voltage. A magnetic transformer (hereinafter transformer) can provide the above-described conversion. CCFL circuits can leverage this transformer in various ways.
For example, FIG. 1 illustrates an exemplary Royer CCFL circuit 100 in which the emitters of NPN transistors Q1 and Q2 are coupled to an inductor whereas the collectors of NPN transistors Q1 and Q2 are connected to opposite ends of the primary winding of a transformer T1. A center tap of transformer T1 is connected to a battery (in this case, a 12 V source). A second primary winding of transformer T1 connects between the bases of NPN transistors Q1 and Q2. Other components, e.g. diodes D1/D2, resistors R1/R2, capacitor C1, and a 200 kHz oscillator, form a regulator circuit to control the current in the CCFL. In this configuration, Royer CCFL circuit 100 functions substantially as a fixed output voltage inverter, wherein its stepped up voltage at node 101 is proportional to the number of turns on the secondary winding divided by the number of turns on the primary winding.
Of importance, this stepped up voltage must be at least the striking voltage of the CCFL. Specifically, before strike (i.e. CCFL as an open circuit), no current flows through capacitor C3 and therefore the voltage across its terminals goes high (i.e. up to the strike voltage). However, after strike, current begins to flow across capacitor C3 and therefore its voltage across its terminals drops to a desired operating voltage.
FIG. 2A illustrates an exemplary direct drive CCFL circuit 200 in which the sources of n-type transistors Q5 and Q6 are coupled to ground whereas their drains are connected to opposite ends of the primary winding of a transformer T3. A p-type transistor Q4 is connected between a center tap of transformer T3 and a battery VBATT. Opposite ends of a secondary winding of transformer T3 are connected to ground and an input terminal of the CCFL. In one embodiment of direct drive CCFL circuit 200, transistors Q5 and Q6 have a duty cycle of 50% (e.g. between 0 and 5 V) whereas transistor Q4 has an adjustable duty cycle between 0% and 100% (e.g. VBATT—7.5 V and VBATT) (see FIG. 2B). In operation, direct drive CCFL circuit 200 effectively functions as a current source output. Specifically, as current is forced through the CCFL, an output voltage 201 will increase to ensure that current continues to flow. At start up of the CCFL, a voltage 201 increases until the CCFL strikes or some component in circuit 200 fails. After the CCFL strikes, the same current will flow in the CCFL, but the voltage will drop to a desired operating voltage.
Direct drive CCFL circuit 200 has several known advantages over Royer CCFL circuit 100. Specifically, direct drive CCFL circuit 200 typically can provide higher efficiency than Royer CCFL circuit 100 with fewer components. Moreover, unlike Royer CCFL circuit 100, direct drive CCFL circuit 200 can advantageously drive multiple CCFL tubes. For example, one known direct drive CCFL circuit having such capability is described in U.S. patent application Ser. No. 10/264,438, entitled “Method And System Of Driving A CCFL, filed on Oct. 3, 2002, which is incorporated by reference herein.
Direct drive circuits do not provide a fixed secondary voltage. Instead, the CCFL, not the driving circuitry, determines the secondary voltage. Because the CCFL effectively sets its own operating point (i.e. provides a self-biasing function), the user does not have to pick an operating voltage for the CCFL, choose the proper capacitance for the ballasting capacitor (e.g. capacitor C3 in Royer CCFL circuit 100), and then modify those values to ensure that enough power will be dissipated in the CCFL to provide adequate illumination. A CCFL-determined secondary voltage is generally considered advantageous because it eliminates the ballasting capacitor.
However, the striking characteristics of a CCFL appear to be a function of both age and temperature. That is, as a tube ages or under very cold conditions, a CCFL may not strike properly. Unfortunately, if the CCFL does not strike within a predetermined time, direct drive CCFL circuit 200 could conclude that a difficult to strike tube is instead a “bad” tube. That is, detect circuitry in direct drive CCFL circuit 200 could conclude that the tube was a safety hazard, and erroneously shut down the CCFL before striking at the new higher voltage.
To “coax” the CCFL into striking properly, some users prefer to hold the voltage across the CCFL at some higher than normal (yet still safe) voltage for a short period of time. Traditional direct drive CCFL circuits, because of their current source nature, are unable to provide a fixed voltage across a CCFL that has not yet struck.
Therefore, a need arises for a method of increasing the usable life from a CCFL as well as improving cold start operation while using a direct drive CCFL circuit.